1. The field of the Invention
This invention relates to novel processes and apparatus for separating dispersed particulate material from fluids containing such material.
2. Description of the Prior Art
Many processes and apparatus for filtering or separating particulate solids from solids/fluid dispersions are known to the art. Perhaps the best known are those processes and apparatus adapted for batch type filtrations. In such filtrations, a fluid having a particulate solid dispersed therein is added to apparatus having porous filtration means which retains the solid but permits the fluid to be transmitted through the filtration means. Pressure and vacuum are usually employed to accelerate the rate of filtration in manners well known to the art. Filtrations of this type are usually slow and cannot be operated in a truly continuous fashion since some accomodation must be made to remove accumulated solids retained by the filtering means; otherwise filtration rates can be diminished to a point where they become ineffective.
Continuous filtration processes and apparatus are also known to the art and involve techniques broadly defined as ultrafiltration and hyperfiltration with the last term usually including reverse osmosis. Ultrafiltration essentially involves the separation of a solvent from a relatively high molecular weight, dispersed or dissolved solute by forcing the solvent through a membrane providing preferential passage for the solvent as opposed to the dissolved solute. Low pressures in the order of about 10 to about 100 psi are usually involved and turbulent flow conditions are required to maintain efficient transfer of solvent vs. solute at the membrane surface.
Hyperfiltration (reverse osmosis) differs from ultrafiltration primarily in the size and molecular weight of the dispersed solute. Essentially, the technique involves the separation of solvent from solutions of low molecular weight salts by forcing the solvent through a membrane under high pressures in the order of from about 400 to 2000 psi. The performance characteristics of the membrane employed in hyperfiltration are much more critical than in the case of the ultrafiltration membrane especially in that the membrane is designed to allow passage of only solvent therethrough.
Membranes employed in the above mentioned ultrafiltration and hyperfiltration techniques are known to the art. They are normally fabricated of porous, film forming plastomeric materials and the membranes are oftentimes disposed on porous substrates or screens for reinforcement. Recently, membranes have been formed on porous substrates in the manner described in "Hyperfiltration Processing of Pulp Mill Sulfite Wastes With a Membrane Dynamically Formed From Feed Constituents" Environmental Science and Technology Vol. 1, Number 12 December 1967 and "Hyperfiltration Studies v. Salt Rejection By Dynamically Formed Hydrous Oxide Membranes" JACS 88:24, Dec. 20, 1967 and "Hyperfiltration" Ind. Eng. Chem., Process Des. Develop., Vol. 9, No. 4, 1970. Additional helpful details relating to hyperfiltration techniques can be found in Volume II of Recent Developments In Separation Science published by Chemical Rubber Company (CRC) Press in 1972 and especially in an article at pages 205-225 entitled "Cross Flow Filtration" by J. D. Henry, Jr.
In these articles, a so called "dynamic" membrane is established by depositing film forming materials such as resins, gels, and polymers on surfaces of porous substrates to provide a membrane useful for separating solvent from dispersed or dissolved solute. The designation of the membranes so formed as "dynamic" may be somewhat misleading since the membranes are actually static, permanent films deposited on the porous substrate surface and regeneration of the membrane is apparently required during operation.
The present invention is addressed to the problems involved in separating particulate solids from solids/fluid dispersions and provides novel processes and apparatus for continuously filtering such dispersions in an especially efficient fashion.